In the past, photonic integrated circuits (PIC), which is also called planar lightwave circuits (PLC) or optical integrated circuits (OIC), are designed using single mode optical waveguide structures. The reasons of using single mode waveguide is to ensure that the whole circuits is within the single mode operation region, therefore, no high order modes will be excited both within the circuits and at the interface of the coupling to the outside world, usually to optical single mode fiber. If the waveguide is not single mode, when the higher order modes are excited, multiple guiding modes will propagate together along the waveguides and each mode carries part of signal power. As the result of the multimode propagation, the signal will suffer significant mode dispersion and random coupling among the modes when discontinuity or structure change occurs. The coupling from the PIC to the end single mode fiber will have large and random loss, making the device not usable in the system.
However, the single mode operation condition of the waveguide is, some time, too restrictive. It will also create problems for the coupling and the achievable circuit density of PICs. A typical example is the silicon-on-insulator (SOI) optical waveguide as shown in FIG. 1.
<11>: Ridge waveguide cross section;
<12>: The substrate of the ridge waveguide (bottom cladding);
<13>: Ridge waveguide core;
<14>: The ridge;
<15>: Slab region of the ridge waveguide;
<16>: Top cladding of the ridge waveguide;
<17>: The layer structure of the starting SOI wafer;
<18>: The silicon layer of the starting SOI wafer;
Due to the strong material index contrast between the core <13> and the cladding (substrate) in SOI (nf=3.48 vs. nc(f)=1.44˜1.8), for large dimension comparable with the regular single mode fiber, the single mode condition must be maintained by the weaker lateral effective index contrast, which is between the slab mode effective indices of the region I and II <15>. The slab region itself, both I and II, are multimode, but the lateral effective index contrast is so weak that all the high order vertical mode of slab region I can not propagate without radiation into the region II.
Since the vertical index contrast in SOI structure is much stronger than other material systems, such as silica (SiO2) and polymer, the SOI waveguide usually ends up have weaker lateral index contrast than silica and polymer based waveguide. It makes the SOI waveguide PIC with large cross section has to have larger bending radius, which results in low circuit density. To achieve a similar bending radius as silica waveguide, the waveguide dimension must be reduced to about half of the single mode fibers, which causes severe problem in coupling. A 3D on-chip taper usually must be used to reduce the coupling loss.
Another problem with SOI based PIC is the splitter junction loss. A typical excess loss of a SOI based waveguide splitter due to the splitter junction is usually ˜1 dB, while the silica based waveguide splitter has only less than 0.5 dB. The larger junction loss is because the high-index contrast between the silicon dioxide (cladding) and the silicon (waveguide core), and the ridge structure itself. When the mode hits the junction, a significant scattering will occur and the field gets an abrupt disturbance. An example is demonstrated in FIG. 2, a 1×2 splitter junction <21>.